1. Field of the Invention
This invention relates to a Group III nitride semiconductor light-emitting device and particularly to a Group III nitride semiconductor light-emitting device which has enhanced the light-extraction efficiency thereof.
2. Description of the Prior Art
The Group III nitride semiconductors (hereinafter abbreviated as “nitride semiconductors”) possess a band gap of a direct transition type of energy corresponding to the visible light through the ultraviolet light region and permit highly efficient light emission and have been consequently reduced to commercial products, such as light-emitting diodes (LEDs) and laser diodes (LDs). Particularly, the realization of white light-emitting diodes due to their combination with fluorescent materials has been expected as a new field for the application of light-emitting diodes.
The output of a light-emitting diode is determined by the product of the internal quantum efficiency having an epitaxial structure or crystallinity, for example, relate thereto multiplied by the light-extraction efficiency having the resorption in the device or shape of the device relate thereto. Among other factors mentioned above, the resorption in the device which affects the light-extraction efficiency arises while the light is repeating collision on a substrate which is impervious to the emitted light or transmission through a light-emitting layer. The total reflection on the surface of the device constitutes itself one factor that has a great effect on the light-extraction construction. It is well known that when a light is advancing from a layer of a large refractivity to a layer of a small refractivity, the part of the light exceeding the critical angle (θc) undergoes total reflection in the interface and no light is extracted to the layer of the small refractivity. In the case of gallium nitride (GaN), for example, since it has a refractivity of 2.4, only the light that enters the escape cone having an apex angle of 24° relative to the direction perpendicular to the surface is extracted to the exterior.
This ratio is 27%. Owing to this effect, therefore, the light-extraction efficiency is greatly restrained. As means to avoid this restraint imposed by the total reflection in the interface on the light-extraction, a method for coarsening the interface (refer, for example, to Japanese Patent No. 2836687) and a method for forking the shape of the device to utilize the escape cone of another surface (refer, for example, to Japanese Patent No 2784537) have been known. For the growth of a nitride semiconductor, the MOCVD (metal-organic chemical vapor deposition) method is predominantly used. The MOCVD method is a method for growing a nitride semiconductor by causing an organic metal to react with a nitrogen source on a substrate. The single crystal of a nitride semiconductor, however, has not yet been produced on a commercial scale. Though the pseudo single crystal substrate obtained by effecting the thick-film epitaxial growth on an Si or GaAs substrate by the HVPE (hydride vapor phase epitaxy) method has been already available in the market, it is extremely expensive. As the substrate for the light emitting diode, therefore, generally a heterogeneous substrate, such as a sapphire (Al2O3) or silicon carbide (SiC) substrate which is stable at high temperature is used.
The sapphire and the SiC as stable substances, however, are known also as substances which are so hard as to allow no easy processing. Consequently, they entail the problem that the process of dividing them into component devices with the object of enhancing the light-extraction efficiency will be rendered difficult. When this division is effected by the mechanical method of dicing, the component devices consequently obtained sustain chippings and cracking so frequently as to render the enhancement of yield difficult to attain. When the division is effected by such means of dry etching as resorts to no mechanical process, it consumes a long time and consequently incurs a serious degradation of productivity.
The mechanical method which resorts to a dicing process is known to form on the processed surface a layer called a shattered layer which interferes with the light-extraction process and, the dry etching as well is known to suffer the electrical and optical properties of the surface to be affected by the exposure to the high energy particles of plasma. The wet etching as a processing method of insignificant damage has been also known (refer, for example, to JP-A HEI 10-190152 and JP-A 2000-68608). The component devices produced by this method, however, have perpendicular sections of division.
This invention, in view of such problems as the decline of the light-extraction efficiency due to the total reflection of light by the nitride semiconductor light emitting device and the decline of the light-extraction efficiency by the shattered layer generated during the mechanical processing method performed on the substrate difficult to process, is aimed at enhancing the light-extraction efficiency of the nitride semiconductor light-emitting device.
This invention has been based on the discovery that the light-extraction efficiency is enhanced by utilizing as a processing means the wet etching process incapable of causing any significance damage, tilting the lateral face of the semiconductor layer in the nitride semiconductor device and utilizing this face as well for the purpose of extracting light.